Mold fill improvements for a molded solder C4 process

ABSTRACT

A mold fill system includes a fill head, at least one solder input port, at least one solder output port, disposed on the leading edge of the fill head body, a mold plate, the fill head body being held near the mold plate, including a plurality of mold pits for injection molded solder and at least one o-ring disposed between the fill head body and the mold plate, for sealing the solder between the fill head body and the mold plate. An input velocity of the solder is sufficient to dislodge air trapped within the mold pits, and a circulartion pump is disposed on either a fill side or a return side of the fill head body to re-circulate the solder between the solder reservoir and the fill head. Optionally, heated plumbing connects the circulation pump, the fill head and the reservoir.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus for creating C4 solder bumps, and more particularly to a method and apparatus for an improved fill head for a molded solder C4 process.

2. Description of the Related Art

CN4P is a technology for creating C4 solder bumps by filling a mold flush to the surface of the mold, putting a wafer in close proximity to the mold, and reflowing the solder to transfer the solder to the wafer. Mold filling is typically accomplished utilizing a solder fill head, which is sealed to the mold plate by an o-ring. The solder fill head is moved slowly across the mold plate to fill the mold.

Several problems exist when filling the mold in this way. First, if the head is moved too fast and/or the seal is too good, the solder traps air, resulting in partially filled mold depressions. Second, if the solder doesn't cool properly before exiting the head in a low oxygen atmosphere, it “balls-up” and can result in a defective fill. Such behavior makes verification of a proper fill difficult. This may also prevent good transfer to the wafer. This “balling-up” failure has been attributed to supercooling of the solder. Finally, oxides and other debris can build up in the solder fill area, causing mold fill defects.

SUMMARY OF THE INVENTION

In view of the foregoing and other exemplary problems, drawbacks, and disadvantages of the conventional methods and structures, an exemplary feature of the present invention is to provide a method and structure in which mold fill improvements are provided for a molded solder C4 process.

In accordance with a first aspect of the present invention, a mold fill system includes a fill head, at least one solder input port, at least one solder output port, a mold plate, the fill head body held next to and moved relative to the mold plate, including a plurality of mold pits for injection molded solder and o-rings disposed between the fill head body and the mold plate (either the head or plate or both may be moved to allow filling of all mold plate pits), for sealing the solder between the fill head body and the mold plate. An input velocity of the solder is chosen to be sufficient to dislodge air trapped within the mold pits when there is trapped air, and a circulartion pump is disposed on either a fill side or a return side of the fill head body to re-circulate the solder between the solder reservoir and the fill head. The circulation pump, the fill head and the reservoir are connected by plumbing, which may be heated if it is of sufficient length to require heating.

In accordance with the present invention, the solder flow is re-circulated through the fill head. The re-circulated flow entrains and removes any air trapped in the mold plate, carrying the air away to the solder reservoir where the air bubbles to the top of the reservoir away from the re-circulating input to the fill head. Additionally, agitation may be applied to the fill head to assist in allowing trapped air to escape.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other exemplary purposes, aspects and advantages will be better understood from the following detailed description of an exemplary embodiment of the invention with reference to the drawings, in which:

FIG. 1 illustrates a fill head cross section of a fill head 100 in accordance with an exemplary embodiment of the present invention;

FIG. 2 illustrates a fill head cross section of a fill head 200 including a solidification zone in accordance with another exemplary embodiment of the present invention; and

FIG. 3 illustrates a fill head cross section of a fill head 300 including a vacuum leading edge zone in accordance with another exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1-3, there are shown exemplary embodiments of the method and structures according to the present invention.

FIG. 1 illustrates a fill head in accordance with an exemplary embodiment of the present invention.

Solder is flowed into the fill head 101 through a solder port 102 and then onto the mold plate 104, which incorporates the mold pits required for injection molded solder, and out through outlet ports 103. Note that many other configurations of inlet and outlet ports are possible. An O-ring or O-rings or other sealing elements 105 seal the solder in the solder fill zone 106.

It is desirable to have an input flow velocity that is sufficient to dislodge any air trapped within the pits on the mold plate 104. The high density of the solder and its resulting flow momentum make it far less likely than other lighter liquids to be diverted from its impinging flow vector by an air bubble. The specific flow necessary to accomplish air removal is determined experimentally for a given head design. The mold fill head 101 is accompanied by a flowing solder loop including a pump (on either the fill or return side), a solder reservoir designed to provide clean solder to the loop, and appropriately heated plumbing connecting the loop components. The reservoir outlet would ideally be at the bottom of the reservoir with the inlet at the top of the reservoir so as to allow the dross to float to the top of the reservoir. The pump (e.g., Wenesco model HTP1V) should be of a size sufficient to provide the necessary flow velocity and pressure. The entire loop including the reservoir may be pressurized (e.g., to approximately 10 PSI) in the case where a particular solder pressure at the fill head is desired.

Alternatively, the exit port 103 or other portion of the fill head 101 may be constrained in a matter that generates pressure due to the applied solder flow. This constraint could be accomplished utilizing a flow cross-sectional area that is smaller than the inlet flow cross-sectional area or through other flow configurations know to those skilled in the art. Changes in flow rate would then cause corresponding changes in head fill zone solder pressure. The specific relationships between flow and pressure for given configurations are computable by those skilled in the art of fluid flow dynamics.

In an exemplary embodiment of the present invention, the fill head 101 is designed so that the solder flows uniformly across the width of the solder fill head 101. Again, fluid dynamic analysis can be applied to this goal. In general, if the flow cross sections entering and exiting the head fill zone are smaller than the sections leading to and from the both the entrance and exit to the fill zone, reasonably uniform behavior can be anticipated. Other configurations designed to create uniform behavior (e.g., including multiple inlet and outlet ports) are possible.

The clean solder and air dislodgement capabilities of a flowing solder based mold fill head can be used in combination with other flow head improvements. FIG. 2 depicts a flowing solder head 200 including a solidification zone 204 designed to promote rapid cooling of the solder in the mold. In this figure, the mold plate is moving from the left to the right. The solder is shown being brought in and removed at an angle other than perpendicular to the head, but such angles are not required.

Solder is brought in through port 201, passes through the solder fill zone which incorporates flow restriction point 202 and out through port 203. The filled mold pits exit through solidification zone 204 where the solder cools and solidifies. This solidification zone may include a cold trailing edge 205 cooled by, for example, convection, contact with a cold element, or utilizing a coolant circulated in contact with elements of the trailing edge through port 206. Note that the cold edge should be thermally isolated from the hot solder zone.

FIG. 3 illustrates an exemplary embodiment of the present invention including a vacuum disposed at the leading edge zone 301 designed to reduce the amount of air initially contained within the mold pits on entry to the solder fill zone. This design allows for lower flow rates designed mainly to reduce the buildup of solder dross within the fill zone.

In accordance with the inventive configuration described above, solder enters at the center of the fill head body and exits at its edges. However, it may be superior to have the solder to enter at the head trailing edge and exit at the leading edge of the fill head. This configuration would maximize the time the mold pits are exposed to the flow designed to entrain the trapped air.

Furthermore, agitation may be added to either the fill head body or the mold plate, or both. Agitation would assist in allowing trapped air to escape, improve the frictional behavior of the o-rings and reduce the level of supercooling. The agitation may be added with any type of vibrating transducer including, but not limited to, a piezoelectric transducer.

In accordance with the exemplary embodiments and features described above, the present invention provides a mold fill system in which no venting is required. That is, air is carried away in the re-circulating solder flow. Thus, seals can be designed with broader constraints. Additionally, the fill head results in purer solder because any dross is carried away in the solder flow. This results in a reduced defect rate. Finally, because the solder is not allowed to sit and cool next to the mold plate, it is possible to run the mold plate colder without the solder sticking to the mold plate.

While the invention has been described in terms of several exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

Further, it is noted that, Applicant's intent is to encompass equivalents of all claim elements, even if amended later during prosecution. 

1. A mold fill system, comprising: a fill head, having a leading edge and a trailing edge; at least one solder input port, for receiving solder from a solder reservoir; at least one solder output port; a mold plate, said fill head being disposed adjacent to said mold plate, said solder flowing through said fill head, through said at least one solder input port and onto said mold plate, said mold plate including a plurality of mold pits for injection molded solder; and at least one seal disposed between said fill head and said mold plate, said at least one seal sealing said solder in a solder fill zone disposed between said fill head body and said mold plate, wherein an input flow velocity of said solder is sufficient to dislodge at least a portion of air trapped within said mold pits, and wherein at least one circulation pump is disposed on at least one of a fill side and a return side of the fill head body to re-circulate said solder between said solder reservoir and said fill head.
 2. The mold fill system according to claim 1, wherein a flow cross-sectional area is smaller than an inlet flow cross-sectional area.
 3. The mold fill system according to claim 1, wherein said solder flows uniformly across a width of said fill head.
 4. The mold fill system according to claim 1, further comprising: a cold element, cooled by a liquid or a gas coolant, disposed on a trailing edge of said fill head to cool and solidify the solder, wherein said cold element is thermally insulated from a remainder of the mold fill system.
 5. The mold fill system according to claim 1, further comprising: a vacuum disposed on the leading edge of said fill head to reduce an amount of air contained within said molded pits.
 6. The mold fill system according to claim 1, further comprising: a vibrating transducer for agitating the molded fill system.
 7. The mold fill system according to claim 1, wherein said at least one circulation pump, said fill head and said reservoir are connected by heated plumbing. 